reboot hangs - what logs to check?

[Craig Hagerman]

I don't often shutdown or restart my ubuntu machine but I have noticed
that it sometimes hangs with a shutdown or reboot. I run this machine
headless, logging in via ssh or VNC so I can't tell what is going on
after I enter "shutdown -r (or h) now". What logs can I look in to
tell what is going on at shutdown or start up to try to solve this
problem?